This invention relates to a system for manufacturing film products using a technique for casting film from a hot mixture of resin and volatile liquid in a pressurized extrusion zone. The system involves a novel process for recovering hot-extruded polyacrylonitrile (PAN) film or the like from an extrusion zone without mechanical seals between ambient pressure and the pressurized extrusion zone.
Prior methods for producing hot extruded PAN films or the like have not been successful when employing coagulant baths, largely due to uneven and irregular coagulation, resulting in microscopic voids or pores, reducing the strength and transparency of the product. Coagulant techniques often produce an outer skin, which may produce contraction differences during solvent removal, resulting in a corrugated or puckered extrusion product.
Production of optically-clear polyacrylonitrile (PAN) film from hot aqueous solutions of PAN has proven difficult in past attempts. It is known that many polymers containing large amounts of acrylonitrile repeating units, especially the homopolymer, are soluble in water at elevated temperature. The high vapor pressure of the aqueous solvent at equilibrium requires a large pressure to be exerted on the hot solution from a high pressure orifice. Abrupt pressure reduction can cause flashing of the solvent, wherein water vaporizes in the cast material and expands rapidly. This phenomenon causes film disruptions, destroying the optical and mechanical properties of the resulting film and introducing discontinuities in the product which adversely affect molecular orientation and gas barrier characteristics.
If the extrudate is cooled immediately as it reaches the extrusion zone, while the zone is maintained above a certain pressure to prevent flashing, the vapor pressure of the volatile solvent is decreased as a function of the film temperature. In U.S. Pat. No. 2,585,444, it is suggested that aqueous PAN extrudates be coagulated under pressure in a receiving cell.
While the expedient of maintaining an extrusion chamber under pressure is effective in preventing solvent flashing, it introduces a problem in recovering the product. Because the film, or similar extruded shape, is made continuously to produce a strip of coagulated polymer, product recovery from the pressurized extrusion zone is difficult. Various mechanical seals, usually including rollers, wiping seals and the like, may be employed to pass the continuous film from high pressure to ambient. Contacting the film with mechanical handling equipment tends to mar its surface, resulting in unacceptable product.
It is known that water can form hydrate with nitrile groups in acrylonitrile polymers. When the polymer consists essentially of PAN homopolymer, about 33 parts by weight of water per hundred parts of polymer are required to give complete hydration. A melt-extrusion mixture containing more than the hydrating amount of water is maintained under at least autogenous pressure to prevent formation of bubbles. Since the aqueous PAN melt composition is formed at relatively high temperatures, i.e. above about 180.degree. C., free water must be highly pressurized to prevent vaporization. Accordingly, extrusion of acrylonitrile polymers containing more than 25% H.sub.2 O, based on total water-acrylonitrile content, are subject to flashing when the pressure is reduced during extrusion. It is an object of the present invention to prevent disruption of the extrudate by vaporization of volatile components by providing a back pressure in the extrusion zone and recovering the extruded shape after cooling at ambient pressure.